The present invention relates to radiological well logging methods and apparatus for investigating the characteristics of subsurface earth formations traversed by a borehole. More particularly, the invention relates to methods and apparatus for measuring the porosity of earth formations in the vicinity of a well borehole by means of neutron well logging techniques.
In the search for liquid hydrocarbons beneath the earth's crust, one of the parameters which must be known about the earth formation is the formation porosity. The porosity, or fractional volume of fluid filled pore space present in and around the rock matrix comprising the earth formation, is needed both to evaluate the formation's commercial production potential, and also to assist in the interpretation of other logs, such as resistivity logs and pulsed neutron logs.
Several techniques have been developed in the prior art to measure earth formation porosity in a borehole environment. One such technique employs a gamma ray source and one or more detectors for measuring the electron density of the earth formations by the amount of gamma ray scattering. Since rock matrix is more dense than pore fluid, this leads to an inferential measurement of the porosity of the formations. Another technique employs an acoustic transmitter and one or more acoustic receivers. The velocity of sound transmission through the formation from the acoustic transmitter to the receivers is then measured. Since rock is more dense than pore fluid, the sound travels faster in less porous rocks than it does in fluid filled pore spaces in more porous earth formations. The measured sound velocity can then be related to the formation porosity.
A third commercial technique which has been employed in the prior art to measure the porosity of earth formations employs a neutron source and any of several types of neutron or gamma ray detectors, depending upon the energy ranges of the neutrons being measured. Because the behavior and interactions of neutrons with matter are quite distinct depending upon their energies, such neutrons are generally divided into at least three distinct energy ranges: fast, epithermal, and thermal. Generally speaking, fast neutrons are those with energies around one Mev (within an order of magnitude or so). Epithermal neutrons have energies around one ev. Thermal neutrons are in thermal equilibrium with their environment and have energies around 0.025 ev. The neutron sources commonly employed all emit neutrons in the fast energy range. Depending upon the formation constituents into which the neutrons are emitted, these energies will then be attenuated at various rates by interactions with the matter in the formation. Generally speaking, hydrogen is the principal agent responsible for slowing down neutrons in an earth formation.
In high porosity formations, fast neutrons are attenuated principally both by inelastic scattering with the rock constituents and by elastic scattering with the hydrogen in the pore fluid. Epithermal neutrons are attenuated by elastic scatter with hydrogen. Inelastic scattering does not affect epithermal neutrons since their energies are below the inelastic reaction threshold energies. Ultimately, the neutrons become thermalized and are absorbed by the nuclei of formation constituents.
In low porosity formations, fast neutrons are attenuated mainly by inelastic scattering with the rock constituents. Epithermal neutrons, however, have a much smaller relative attenuation cross-section since there is very little pore fluid, and hence hydrogen, present.
A common neutron porosity logging technique is one which employs either a neutron or gamma ray detector which is sensitive to the intensity of the thermalized neutrons at some point removed from the neutron source. Then, in a formation containing a larger amount of hydrogen than is present in low porosity formations, the neutron distribution is more rapidly slowed down, and is contained in the area of the formation near the source. Therefore, the counting rates in remote thermal neutron sensitive detectors located several inches or more from the source will be suppressed. In lower porosity formations which contain little hydrogen, the source neutrons are able to penetrate farther. Hence, the counting rates in the more remote detector or detectors are increased. This behavior may be directly quantified into a measurement of the porosity by well established procedures. Combinations (e.g., ratios) of the count rates in two or more detectors at different distances from the neutron source are sometimes employed for improved results. In such a case, the ratio of the near spaced to the far spaced detector count rates is observed to increase as porosity increases. The measurement technique is thus essentially spatial, relying upon variations in the spatial distribution of the neutrons.
Such commercial methods utilizing thermal neutron measurements have generally not proven to be as accurate as desirable due to diameter irregularities of the borehole wall, variation of the properties of different borehole fluids, the irregular cement annulus surrounding the casing in a cased well borehole, and the properties of different types of steel casings and formation lithologies which surround the borehole. For example, since chlorine has a high absorption cross-section for thermal neutrons, the thermal neutron distribution surrounding a prior art source and detector pair sonde can be affected by the chlorine content of the borehole fluid. Similarly, lithological properties of the earth formations in the vicinity of the borehole, such as the shale or boron content of these formations, can affect the measurement of thermal neutron populations. Also, thermal neutron measurements are very sensitive to the formation matrix type, i.e., whether the formation matrix is sand, limestone, or dolomite.
Improved methods and apparatus for such measurements have been suggested which employ epithermal and/or fast neutrons. As described above, these are less sensitive to formation lithology effects, and are not affected by small concentrations of strong thermal neutron absorbers such as chlorine or boron. One prior art fast/epithermal neutron technique which does not rely on purely spatial concepts is described in U.S. Pat. No. 4,134,001 (Smith, Jr. et al., issued Jan. 9, 1979). As disclosed in greater detail therein, the method and apparatus employ a ratio measurement of fast/epithermal neutron flux in two detectors approximately equally spaced from a fast neutron source. (Differences in the detector-source distances are supposed to be compensated, as by weighting the ratio.) Porosity is then determined as a function of changes in the shape of the overall neutron spectrum between fast and epithermal energies. ("Spectrum", in this reference, refers to the gross count rate differences between fast and epithermal neutrons. Specific spectra of the fast and of the epithermal neutrons are not themselves utilized or taken.)
While such techniques as described above are effective, it would be a distinct advantage and improvement if a significantly greater dynamic range could be realized than is provided by these prior art techniques alone. Accordingly, a need still remains for improved methods and apparatus for measuring formation porosity using neutron measurement techniques. Preferably such methods and apparatus will lend themselves to crossplotting with other independent porosity measurements (e.g., sonic, density, and other neutron).